Currently available flue gas desulfurization processes use a number of reagents and performance additives to scrub sulfur dioxide from the flue gas and to enhance or control scrubber operation. Reagents, such as limestone or lime, are added to the scrubber as a slurry to serve as a source of the alkalinity for scrubbing the sulfur dioxide. Organic acids may be added to the alkaline slurries to enhance scrubber performance in removing sulfur dioxide. Oxidation may be forced by the addition of oxidation air or inhibited by the addition of thiosulfate. Other additives may also be used to affect various aspects of the flue gas desulfurization process. With the exception of lime or limestone, the addition rates of most of these additives are usually not automatically controlled in response to a feedback control logic from an on-line analytical measurement. Lime or limestone addition rates are typically controlled in response to measurement of the pH of the process slurry. However, unless some type of control is exerted over reagent or additive addition, it is difficult to adjust reagent or additive usage to the level required to efficiently achieve the required sulfur dioxide removal or the desired benefit of the additive.
The objective in controlling additive or reagent addition is generally to minimize reagent usage while concurrently achieving the required sulfur dioxide removal or the maximum benefit of the additives. In the past, additive addition rates have typically been set at fixed rates which were determined from past plant operating experience. However, this practice has not met the control objective of maintaining the concentration of the flue gas desulfurization process additive at a level required to achieve the needed benefit of the additive. This approach provides no capability for adjusting the additive addition rate in response to changing process conditions or as needed to maximize the benefit of the additive while simultaneously minimizing usage and cost.
The prior art has suggested methods and systems for measuring various flue gas desulfurization system parameters and making the necessary adjustments to enhance sulfur dioxide removal. For example, U.S. Pat. No. 5,168,065 to Jankura et al. discloses the measurement of pH values as a basis for adjusting the amount of air supplied to a flue gas desulfurization reaction tank to maximize oxidation. Oxidation is measured indirectly by directly measuring the pH of the absorber circulating slurry. A feedback control adjusts the amount of air to the tank.
U.S. Pat. No. 4,677,077 to Onizuka et at. describes a method for continuously measuring the calcium carbonate concentration in a slurry such as an absorption liquid slurry used in a wet lime flue gas desulfurization process. This method may effectively measure the slurry calcium carbonate concentration. However, it is not suggested that the measurement method disclosed by Onizuka et al. could be used in conjunction with an on-line automatic monitoring and control system to adjust the conduct of a flue gas desulfurization process. Neither is it suggested that an automatic on-line monitoring and control system which monitors flue gas desulfurization process indicators in slurry flitrate could be integrated with such a slurry measurement system.
U.S. Pat. No. 4,582,692 to Hamanaka et al. discloses a flue gas desulfurization process wherein pH is automatically detected and regulated in connection with a number of pumps which are then set according to the amount of exhaust load in the absorber to control lime and limestone addition.
The control of the circulating flow rate of absorbing liquid slurry and slurry flow volume in flue gas desulfurization processes has also been disclosed by the prior art. Japanese Patent Publication Nos. 2266117 and 361234913 disclose controllers that accomplish this. Japanese Patent Publication No. 3127612 discloses an on-line desulfurization process performance indicator that detects the inlet sulfur dioxide concentration, the outlet sulfur dioxide concentration, pH and recirculating flow rate to control the circulating flow rate of absorbing liquid.
The prior art does not suggest the use of automatic on-line analyzers for monitoring and controlling the addition of flue gas desulfurization process performance enhancement additives or process control additives other than oxidation air and carbonate concentration. A need exists for an automatic on-line analysis system for analyzing filtered and unfiltered slurry in a flue gas desulfurization system to measure and control, as necessary, the addition rate of flue gas desulfurization process control or performance enhancement additives and to monitor key process control indicators.